This invention relates to a glow plug utilized to preheat the sub- or auxiliary combustion chamber or the main combustion chamber of a diesel engine.
The starting characteristic of a diesel engine is generally poor at low temperature so that it is the common practice to increase the suction temperature by installing a glow plug in the sub-combustion chamber or main combustion chamber and by passing electric current through the glow plug. The glow plug is also used as a source of ignition for improving the starting characteristic of the diesel engine. As the glow plug, the so-called sheathed type glow plug has been used in which a helical heating wire made of iron, chromium or nickel or alloys thereof is packed in a heat resistant electric insulating powder contained in a sheath made of heat resistant metal. The sheathed type glow plug can be used over a long time in high temperature gas in the sub- or main combustion chamber.
With the sheathed type glow plug described above, since the sheath is heated indirectly by the heating wire it takes a long time to increase the temperature of the glow plug to the operating temperature. More particularly, the heat generated by passing electric current through the heating wire is transmitted to the sheath through the heat resistant electric insulating powder, it takes a long time before the sheath becomes red hot. Moreover, the efficiency of heat transmission is low. Thus for example, it takes several of tens seconds until the temperature of the plug is increased to 900.degree. C., for example necessary to start the engine, thus failing to rapidly start the diesel engine.
The temperature rise time can be decreased by supplying a large electric power at the initial stage of energization so as to cause the heating wire to quickly generate heat, but such measure is liable to melt the heating wire or to damage the sheath by high temperature. This also affects the battery and electric circuit and in an extreme case melts a fuse. To prevent these problems, it is necessary to provide a temperature control device for the heating wire which increases the cost of the preheating device.
In recent years, diesel engines are abundantly used for general purpose motor cars, and in order to make the diesel engines to be comparable with gasoline engines it is desired to develop a quick heating type glow plug for improving the starting characteristic.
To meet such requirement, various improvements have been made including the selection of the material for the heating wire, and to improve the temperature rise characteristic by supplying a large electric power. Glow plugs have been developed in which heating wires made of two types of materials having different resistance temperature coefficients are used as disclosed in British Patent No. 1,376,817 and Japanese Laid Open Patent Specification No. 109,538 of 1979.
Although the glow plug utilizing heating wires made of two types of materials can quickly raise the temperature, its cost of manufacturing is high and the heat generating characteristic is not yet satisfactory, so that there are many points to be improved.
We have made exhaustive researches and investigations for more efficiently utilizing the advantage of using a heating wire made of two types of materials and found that in order to obtain a glow plug having an optimum characteristic, it is necessary to set the length of two heating wires especially the front heating wire and the resistance ratio of two heating wires in predetermined ranges.
There are many factors that contribute to the inprovement of the heating characteristics of the glow plug of the type described above. Among these factors may be mentioned the materials of the two types of the heating members, diameter of the wire, outer diameters of the heating members, the length thereof, resistance value at room temperature (20.degree. C.); and the material, wall thickness, outer diameter and length of the sheath, as well as the voltage applied to the glow plug. It is difficult to determine which one of these many factors constitutes the basic factor that determines unique characteristic of the glow plug.
In order to obtain a satisfactory heating characteristic of the glow plug it is necessary to select various factors described above by taking into consideration the time necessary to reach 900.degree. C. necessary to start the diesel engine, that is rapid heating characteristic, and limiting the saturation temperature to about 1000.degree. C.-1100.degree. C. thereby ensuring the durability of the heating members.
Furthermore, we have found that in the prior art glow plug, since two types of the heating members are disposed too closely, the rear heating member acting as the heating element is influenced by the heat generated by the front (first) heating member acting as an heating element so that the resistance value of the rear (second) heating member increases rapidly, thus shortening the supply time of large power to the front heating member at the initial stage of current supply.
More particularly, in the prior art glow plug of this type, a heating member having a lower resistance temperature coefficient is disposed on the front side, while a heating member having a higher resistance temperature coefficient is disposed on the rear side and wherein the resistance variation caused by the temperature variation of the rear side heater is used to supply large power to the front side heating member immediately after starting energization so as to rapidly red heat the front side heating member. However, since both heating members are too closely disposed the temperature of the rear side heating member is quickly raised in a short time due to the effect of heat from the front side heating member whereby the electric power supplied to the front side heating member is limited. For this reason the prior art glow plug of this type can not manifest rapid heating characteristic.